The bearings of turbo-driven machinery such as, for example, large turbine-generators, are designed to operate with a continuous flow of lubricating oil so that the load of the shaft of rotor is borne by a hydrodynamic oil film formed between the shaft and the bearing surface. Large quantities of oil are required to pass through the bearings for this purpose and for the additional purpose of carrying away excess heat to prevent the bearings from overheating.
The conventional method of supplying lubricating oil to these machines is to provide an oil reservoir and a primary pumping system, powered by the rotating shaft of the turbine, to continuously pump oil from the reservoir to the bearings. As exemplified by U.S. Pat. No. 2,440,980 to Sheppard, these pumping systems utilize centrifugal pumps, the main one of which is coupled mechanically to the turbine shaft. The delivery pressure of these systems (i.e., of the centrifugal pumps) varies essentially with the square of the shaft speed, so that, during turbine shutdown, when the rotor is coasting down in speed, a point will be reached at which oil can no longer be delivered from the oil tank to the bearings. The system thus "collapses" during coast down. Typically, for a 3600 RPM machine the collapse speed may occur between 2000 and 2500 RPM. Similarly, during turbine startup, the pumping system will not prime itself since the oil reservoir is located below the turbine to permit gravity to be used to drain the bearings.
To make up for these shortcomings, motor driven auxiliary pumps are used to maintain the bearing oil flow at turbine speeds below the collapse speed and to prime the main centrifugal pump for startup. This dependence on auxiliary pumps requires a system of high reliability, however, to ensure that oil is pumped to the bearings during all operating phases of the turbine. In this regard, provision is made for backup emergency oil pumps which are powered by separate, direct current power sources and which are brought into action if the auxiliary pumps or their power sources fail. Overall, these systems are quite complex and despite great care in the provision of backup measures, failures still occur with some regularity.
With a failure of the lube oil pumping system resulting in loss of oil flow to the bearings, not only are the bearings themselves damaged but there is a potential for damage to the turbine rotor itself. Without lubrication, the journal bearing face metal (usually babbit metal) shortly becomes destroyed allowing the rotating shaft to sink somewhat into the supporting metal. This causes a loss of clearance around the rotor spill strips and so forth until the rotor ultimately begins to rub upon stationary parts. The damage can be very severe not only in terms of physical harm to the machine but also in economic terms because of lost power generating capacity.
In an earlier era, when turbomachines (such as turbine-generators) were smaller, mechanical drive oil delivery systems were used. These systems included a bevel or worm gear arrangement, driven by the turbine shaft, and a vertical shaft leading down to the oil tank where the shaft was coupled to a gear pump. Such systems became impractical with the advent of larger turbines, however. Also impractical with larger turbines, is the use of close-mounted oil tanks and self-priming positive displacement pumps driven by the turbine shaft. If the turbine is large, use of a close mounted tank makes it very difficult to rely on gravity to drain the more remote bearings. In addition, the large oil tank required for a larger turbine complicates installation close under the turbine.
An alternative system, somewhat widely used and satisfactory in combination with some smaller turbine-generators, provides ac motor driven positive displacement or centrifugal oil pumps for which the ac power is drawn from the main power generator. During startup and shutdown, however, this source of power is not operational and auxiliary power sources must be relied upon. Those problems described above therefore arise.
Accordingly, it is among the objects of the present invention to provide, for use with a turbomachine such as a turbine-generator, a lubricating oil pumping system which is powered by the turbine being lubricated and which is capable of maintaining a continuous flow of lubricating oil sufficient for bearing lubrication down to turbine speeds below which the turbine may be operated for short periods of time without bearing lubrication.
Generally, it is an object of the invention to provide a lubricating oil pumping system, for a turbomachine, which is operatively driven by the turbine main shaft, which has a very low pumping collapse speed, which is simple and straightforward, which is self priming, and which is able to provide bearing lubrication without reliance upon backup pumping systems.